Fuel cells which are known in the art can, in general, be grouped into two classes, depending on the operating temperature of the cell: (1) high temperature fuel cells which are operated at about 600.degree. C. to 700.degree. C. using solid electrolytes or fused salt electrolytes and (2) low temperature fuel cells which are operated at below about 200.degree. C. using either alkaline electrolytes, such as potassium hydroxide, or acid electrolytes, such as sulfuric acid or phosphoric acid.
The alkaline electrolyte cell has the disadvantage that CO.sub.2 cannot be tolerated because it reacts with the alkaline solution and results in the precipitation of solid carbonates which physically block catalyst sites on the anode (the fuel electrode), thereby reducing electric power generation. This intolerance of alkaline cells to CO.sub.2 restricts the choice of both the oxidant and the fuel. For example, economically, air is preferred over pure oxygen as the oxidant, but if air is used, it must first be scrubbed to remove CO.sub.2. The choice of fuel is restricted to either pure hydrogen or pure hydrazine. If a carbonaceous fuel is used, means must be provided to prevent the CO.sub.2 by-product from contacting the electrolyte.
Since the acid electrolyte cell can tolerate CO.sub.2, unscrubbed air, as the oxidant, and a carbonaceous material, as the fuel, can be used. Sulfuric acid or phosphoric acid is most commonly used as the electrolyte. The phosphoric acid cell is theoretically the most suitable because, in addition to its tolerance of CO.sub.2, it supplies the heat required for the vaporization of water (The introduction of "From Electrocatalysis to Fuel Cells" edited by G. Sandstede, Univ. of Washington Press, Seattle, Washington, 1972).
The electrocatalyst used as an electrode in either an alkaline or acid cell should exhibit high specific activity, long life and low polarization loss. Various types of platinum electrodes have been used as both anodes and cathodes in alkaline and acid electrolyte fuel cells. However, platinum is a poor catalyst for the reduction of oxygen. The exchange current density, which is a measure of the rate of an electrochemical reaction, for the reduction of oxygen on platinum is 4 or 5 orders of magnitude smaller than the exchange current density for the oxidation of hydrogen on platinum. The search for better electrocatalysts for oxygen reduction thus far appears to have been successful only for alkaline electrolyte cells. Meadowcroft (Nature (London) 226, 847 (1970)) discloses that La.sub.0.8 Sr.sub.0.2 CoO.sub.3 possesses higher activity than platinum in the reduction of oxygen in potassium hydroxide. Matsumoto, Yoneyama and Tamura (Chem. Lett., 661 (1975); J. Electroanal. Chem. 80, 115 (1977); ibid. 83, 167 (1977)) disclose that the activity of perovskite oxides based on LaNiO.sub.3 in the reduction of oxygen in sodium hydroxide is comparable to that of platinum. None of these oxygen reduction electrocatalysts which are useful with basic electrolytes is stable in acid. Because of its stability in acid, platinum is generally chosen for the oxygen reduction cathode in an acid cell even though it is not a particularly good electrocatalyst for reducing oxygen and even though platinum is not entirely stable. As to the latter, it is known that the surface area of an oxygen reduction platinum cathode in a phosphoric acid fuel cell decreases with time, thus causing lower cell output power (Kunz, "Proceedings of the Symposium on Electrode Materials and Processes for Energy Conversion and Storage," Vol. 77-6, edited by McIntyr et al., The Electrochemical Society, Princeton, New Jersey, p. 607).
U.S. Pat. No. 3,663,181 discloses platinum-metal oxides of orthorhombic structure and having the prototype formula Pt.sub.3 MO.sub.6 wherein M is Mn, Fe, Co, Ni, Cu, Zn, Mg or Cd; the process for preparing these platinum-metal oxides by heating appropriate metal oxdides in an oxygen-rich atmosphere at above 500.degree. C. and a pressure of at least 100 atmospheres; and the use of these platinum-metal oxides as catalysts for the hydrogenation of ethylene. Mueller and Roy, J. Less-Common Metals, 19, 209 (1969) disclose the preparation of Pt.sub.3 CuO.sub.6 by reacting cupric oxide and platinum black at 200 atmospheres of oxygen and 890.degree. C. Hoekstra, Siegel and Gallagher, Advan. Chem. Ser., No. 98, 39 (1971), disclose the preparation of MPt.sub.3 O.sub.6, wherein M is Co, Ni, Cu, Mg, Zn, Cd or Hg, by heating a 1:1 molar ratio mixture of the appropriate oxides at 800.degree. C. and 40 kb pressure for one hour. Cahen, Ibers and Wagner, Inorg. Chem. 13, 1377 (1974) disclose the preparation of Cd.sub.X Pt.sub.3 O.sub.6 from CdCO.sub.3 and (NH.sub.4).sub.2 PtCl.sub.6, or platinum black, under one atmosphere of oxygen at 580.degree. C. for one week. British Pat. No. 1,134,111 discloses the use of a homogeneous mixture of a platinum group oxide and an oxideof nickel, cobalt, iron or copper, in a weight ratio of not less than 3:1, in catalyzed reactions.